When fixing a seat belt buckle on a longitudinal guide rail for a motor vehicle seat there is the problem that in the event of a crash very large forces can act on the buckle when a vehicle occupant is restrained by a seat belt connected to this buckle. These forces have—in relation to a longitudinal guide rail for a vehicle seat installed in the vehicle—a component directed vertically upwards (perpendicular to the longitudinal guide rail) and a component directed inwards (perpendicular to the longitudinal guide rail and to the vertical), corresponding to the traction forces which act on the seat belt connected to the belt buckle as the belt restrains the occupant seated on the corresponding vehicle seat. Such forces can in particularly with serious accidents lead to the retaining bracket through which the belt buckle is fixed to the guide rail of the longitudinal guide becoming detached from the corresponding rail if the fixing points do not withstand the forces which arise.